A power semiconductor device such as a switching device and a diode is used in a power converter circuit such as a switching supply and an inverter. The power semiconductor device is required to have high breakdown voltage and a low ON-resistance. There is a trade-off relationship between the breakdown voltage and the ON-resistance, depending on device material. In a power semiconductor device using silicon as a main device material, a marginal low ON-resistance has been realized by the progress of technical development up to now.
In order to further reduce the ON-resistance of a power semiconductor device, a device material is required to be changed. Thus, a wide-band gap semiconductor including a nitride semiconductor such as gallium nitride (GaN), aluminum gallium nitride (AlGaN), or silicon carbide (SiC) is used as a switching device material. According to this constitution, a trade-off relationship determined by a material can be improved, and the ON-resistance can be dramatically reduced.
Among devices using a nitride semiconductor such as GaN and AlGaN, as a device in which a low ON-resistance is easily obtained, there is a heterojunction field effect transistor (HFET: Hetero-structure Field Effect Transistor) using an AlGaN/GaN hetero-structure. The HFET realizes the low ON-resistance by utilizing, as a channel, high-mobility and highly-concentrated two-dimensional electron gas (2DEG) generated on an AlGaN/GaN hetero-interface by a piezoelectric polarization. Consequently, a device with a small chip area but a low ON-resistance can be obtained. Since a device capacity is reduced due to the small chip area, a device suitable for high-speed switching operation can be obtained.
However, in fact, when HFET is switched at high speed, switching noise due to a surge voltage/current according to a parasitic inductance is easily generated. When the switching noise is propagated to a gate line, there occur problems such as occurrence of loss and destruction of a device due to malfunction of the HFET.